Typical photographic elements use silver halide emulsions, the silver halide having a native sensitivity to ultraviolet UV radiation ("UV"). UV radiation is usually regarded as anything less than about 400 nm. Such UV sensitivity is usually undesirable in that it produces an image on the photographic element which is not visible to the human eye. In addition, in the case of color photographic elements, in particular, color dye images formed on the light sensitive emulsion layers by color development easily undergo fading or discoloration due to the action of UV. Also, color formers, or so-called couplers, remaining in the emulsion layers are subject to the action of UV to form undesirable color stains on the finished photographs. The fading and the discoloration of the color images are easily caused by UV of wavelengths near the visible region, namely, those of wavelengths from 300 to 400 nm. For the foregoing reasons, photographic elements typically incorporate a UV absorbing material particularly in an upper layer.
Many types of UV absorbing materials have been described previously, and include those described in U.S. Pat. Nos. 3,215,530, 3,707,375, 3,705,805, 3,352,681, 3,278,448, 3,253,921, and 3,738,837, 4,045,229, 4,790,959, 4,853,471, 4,865,957, and 4,752,298, and United Kingdom Patent 1,338,265. Known UV absorbing materials often have many undesirable characteristics. For example, they tend to color and form stains due to their insufficient stability to UV, heat, and humidity. Also, a high-boiling organic solvent is usually required for the emulsification of the UV absorbing agents, which softens the layer and substantially deteriorates interlayer adhesion. In order to prevent these problems, a large amount of gelatin has been used in the layer containing the UV absorbent, resulting in a layer which may be unstable. Alternatively, a separate gelatin protective layer was provided over the UV absorbent containing layer. Such approach results in an undesirable thickening of the element. Furthermore, previously known UV absorbing agents, when provided in the uppermost layer of a photographic element, often migrate and crystallize at the surface of the layer. Thus, a gel overcoat would be used to minimize this undesirable blooming phenomenon. Furthermore, the droplets of such UV absorbing materials, when prepared by the conventional emulsification method described above, usually have particle sizes greater than 200 nm thereby producing light scattering with resulting deterioration of the element's photographic properties. The toxicity of such UV absorbing agents has also become an important issue recently.
It is known that polymer latexes obtained by polymerization of UV absorbing monomers can be utilized as a UV absorbing agents which do not have such disadvantages. Two processes for adding polymeric UV absorbing agents in the form of a latex to hydrophilic colloid compositions, such as gelatin/water compositions, have been known. The first process comprises adding a latex prepared by emulsion polymerization directly to a gelatin-containing silver halide emulsion. Emulsion polymerization is well known in the art and is described in :(1) F. A. Bovey, Emulsion Polymerization, Interscience Publishers Inc., New York, 1955. (2) C. Schildknecht and I. Skeist, Polymerization Process, pp.143-197, Wiley-Interscience Publication, NY, 1977 and (3) R. Fitch, Polymer Colloid II, Plenum Press, NY, 1980. This is the most desirable process because the polymer latex can be prepared in one step.
However, the possibility of using emulsion polymerization is limited by the fact that the monomers need be dispersed in liquid form while most UV absorbing monomers are solid at room temperature and have very low water solubilities.
As the term emulsion polymerization implies, it is first necessary for the monomer to be dispersed in the form of oil droplets in the aqueous phase according to the theory that Harkins proposed on the emulsion polymerization of styrene, and that qualitatively complies with the emulsion polymerization of most other liquid monomers (J. Am. Chem. Soc. 69, 1428(1947); J. Polymer Sci. 5, 217(1950)). When a water-immiscible, organic monomer is dispersed in water in the presence of a surface-active emulsifying agent, the monomer is mainly dispersed in droplets of a diameter of about 1.mu. and these droplets are stabilized by emulsifier (or surfactants). A little amount of the monomer, however, is solubilized in micelles formed by emulsifier. Radicals formed by decomposition of a polymerization catalyst ,for example persulfate, initiate the polymerization solely in the micelies, which are thereby transformed progressively in polymer particles swollen by monomer. The monomer in these particles is gradually used up as polymerization proceeds but is continuously renewed by diffusion from the monomer droplets through the aqueous phase. The monomer droplets are thus acting as highly dispersed reservoirs of monomers.
From the Harkins theory, it is not difficult to understand that solid monomers, with melting point higher than the polymerization temperature and with relatively low solubility in water, cannot be transformed to polymer latex by emulsion polymerization because they cannot diffuse through the aqueous phase in order to supply the monomer to the loci of the polymerization.
To solve the foregoing problem, an inert organic solvent has been used to dissolve the UV absorbing monomer and the solution was added to the polymerization vessel containing water, surfactant, and initiator to start the polymerization. This method have been described in EP 0 190 003, U.S. Pat. Nos. 3,761,272, 3,813,255, 4,431,726, 4,455,368, and 4,645,735. Although polymer latexes can be prepared by this method, there are many disadvantages. First, the inert solvent usually must be removed by distillation or diafiltration after the polymerization. This increases the manufacturing cost and adds a waste stream to the process. Second, the inert solvent interferes with the function of surfactants used in emulsion polymerization and reduces the colloidal stability of the polymer latex. Latex then may coagulate during the polymerization or coagulate upon storage. Third, the % solid of the polymer latex is low because it is limited by the solubility of the UV absorbing monomer. Additionally, the use of surfactant for the stabilization of polymer latex have some undesirable effects on the photographic materials. First, some surfactants interact strongly with gelatin and cause coating difficulty. Second, some surfactants have adverse effects on the photographic materials, such as yellowing or stain.
The second way of forming a UV polymer latex is by dispersing polymeric UV absorber made by solution polymerization in an aqueous medium. In this method, all of the ethylenically unsaturated monomers, including UV monomers and comonomer, are dissolved in an organic solvent and an free radical initiator is added to initiate the polymerization (see Billmeyer, Jr., Textbook of Polymer Science, Wiley-Interscience (1971). The polymer obtained is isolated by precipitation, redissolved in an auxiliary solvent (i.e. ethyl acetate) and a high-boiling coupler solvent, and subsequently dispersed in aqueous solution containing gelatin and surfactant to form a latex.
The disadvantages of the foregoing method is that the auxiliary solvent used in the dispersion must be removed either by evaporation or washing after the dispersion is made. The dispersion procedure is energy consuming and the dispersion formed needs to be stored in cold condition to prevent aggregation. The particle size of the dispersion made from this process is usually greater than 0.2 .mu.m which tends to increase light scattering in a photographic element and reduce the sharpness of the image which can be produced. Additionally, the use of high boiling permanent solvent for the dispersion tends to damage the physical properties of the photographic materials, such as dry and wet scratch resistance and mushiness. Polymeric UV absorbers prepared by this method have been described in U.S. Pat. Nos. 4,496,650, 4,431,726, 4,464,462 and 4,645,735.
It would be desirable then, to have a method of making photographic polymeric UV absorbing compounds, and photographic elements containing them, which improves over one or more of the foregoing disadvantages of the known polymerization methods for forming polymeric UV absorbing compounds.